977 lines · cpp
1//===- ConstantHoisting.cpp - Prepare code for expensive constants --------===//2//3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.4// See https://llvm.org/LICENSE.txt for license information.5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception6//7//===----------------------------------------------------------------------===//8//9// This pass identifies expensive constants to hoist and coalesces them to10// better prepare it for SelectionDAG-based code generation. This works around11// the limitations of the basic-block-at-a-time approach.12//13// First it scans all instructions for integer constants and calculates its14// cost. If the constant can be folded into the instruction (the cost is15// TCC_Free) or the cost is just a simple operation (TCC_BASIC), then we don't16// consider it expensive and leave it alone. This is the default behavior and17// the default implementation of getIntImmCostInst will always return TCC_Free.18//19// If the cost is more than TCC_BASIC, then the integer constant can't be folded20// into the instruction and it might be beneficial to hoist the constant.21// Similar constants are coalesced to reduce register pressure and22// materialization code.23//24// When a constant is hoisted, it is also hidden behind a bitcast to force it to25// be live-out of the basic block. Otherwise the constant would be just26// duplicated and each basic block would have its own copy in the SelectionDAG.27// The SelectionDAG recognizes such constants as opaque and doesn't perform28// certain transformations on them, which would create a new expensive constant.29//30// This optimization is only applied to integer constants in instructions and31// simple (this means not nested) constant cast expressions. For example:32// %0 = load i64* inttoptr (i64 big_constant to i64*)33//===----------------------------------------------------------------------===//34 35#include "llvm/Transforms/Scalar/ConstantHoisting.h"36#include "llvm/ADT/APInt.h"37#include "llvm/ADT/DenseMap.h"38#include "llvm/ADT/SmallPtrSet.h"39#include "llvm/ADT/SmallVector.h"40#include "llvm/ADT/Statistic.h"41#include "llvm/Analysis/BlockFrequencyInfo.h"42#include "llvm/Analysis/ProfileSummaryInfo.h"43#include "llvm/Analysis/TargetTransformInfo.h"44#include "llvm/IR/BasicBlock.h"45#include "llvm/IR/Constants.h"46#include "llvm/IR/DataLayout.h"47#include "llvm/IR/Dominators.h"48#include "llvm/IR/Function.h"49#include "llvm/IR/InstrTypes.h"50#include "llvm/IR/Instruction.h"51#include "llvm/IR/Instructions.h"52#include "llvm/IR/IntrinsicInst.h"53#include "llvm/IR/Operator.h"54#include "llvm/IR/Value.h"55#include "llvm/InitializePasses.h"56#include "llvm/Pass.h"57#include "llvm/Support/BlockFrequency.h"58#include "llvm/Support/Casting.h"59#include "llvm/Support/CommandLine.h"60#include "llvm/Support/Debug.h"61#include "llvm/Support/raw_ostream.h"62#include "llvm/Transforms/Scalar.h"63#include "llvm/Transforms/Utils/Local.h"64#include "llvm/Transforms/Utils/SizeOpts.h"65#include <cassert>66#include <iterator>67#include <tuple>68#include <utility>69 70using namespace llvm;71using namespace consthoist;72 73#define DEBUG_TYPE "consthoist"74 75STATISTIC(NumConstantsHoisted, "Number of constants hoisted");76STATISTIC(NumConstantsRebased, "Number of constants rebased");77 78static cl::opt<bool> ConstHoistWithBlockFrequency(79 "consthoist-with-block-frequency", cl::init(true), cl::Hidden,80 cl::desc("Enable the use of the block frequency analysis to reduce the "81 "chance to execute const materialization more frequently than "82 "without hoisting."));83 84static cl::opt<bool> ConstHoistGEP(85 "consthoist-gep", cl::init(false), cl::Hidden,86 cl::desc("Try hoisting constant gep expressions"));87 88static cl::opt<unsigned>89MinNumOfDependentToRebase("consthoist-min-num-to-rebase",90 cl::desc("Do not rebase if number of dependent constants of a Base is less "91 "than this number."),92 cl::init(0), cl::Hidden);93 94namespace {95 96/// The constant hoisting pass.97class ConstantHoistingLegacyPass : public FunctionPass {98public:99 static char ID; // Pass identification, replacement for typeid100 101 ConstantHoistingLegacyPass() : FunctionPass(ID) {102 initializeConstantHoistingLegacyPassPass(*PassRegistry::getPassRegistry());103 }104 105 bool runOnFunction(Function &Fn) override;106 107 StringRef getPassName() const override { return "Constant Hoisting"; }108 109 void getAnalysisUsage(AnalysisUsage &AU) const override {110 AU.setPreservesCFG();111 if (ConstHoistWithBlockFrequency)112 AU.addRequired<BlockFrequencyInfoWrapperPass>();113 AU.addRequired<DominatorTreeWrapperPass>();114 AU.addRequired<ProfileSummaryInfoWrapperPass>();115 AU.addRequired<TargetTransformInfoWrapperPass>();116 }117 118private:119 ConstantHoistingPass Impl;120};121 122} // end anonymous namespace123 124char ConstantHoistingLegacyPass::ID = 0;125 126INITIALIZE_PASS_BEGIN(ConstantHoistingLegacyPass, "consthoist",127 "Constant Hoisting", false, false)128INITIALIZE_PASS_DEPENDENCY(BlockFrequencyInfoWrapperPass)129INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)130INITIALIZE_PASS_DEPENDENCY(ProfileSummaryInfoWrapperPass)131INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)132INITIALIZE_PASS_END(ConstantHoistingLegacyPass, "consthoist",133 "Constant Hoisting", false, false)134 135FunctionPass *llvm::createConstantHoistingPass() {136 return new ConstantHoistingLegacyPass();137}138 139/// Perform the constant hoisting optimization for the given function.140bool ConstantHoistingLegacyPass::runOnFunction(Function &Fn) {141 if (skipFunction(Fn))142 return false;143 144 LLVM_DEBUG(dbgs() << "********** Begin Constant Hoisting **********\n");145 LLVM_DEBUG(dbgs() << "********** Function: " << Fn.getName() << '\n');146 147 bool MadeChange =148 Impl.runImpl(Fn, getAnalysis<TargetTransformInfoWrapperPass>().getTTI(Fn),149 getAnalysis<DominatorTreeWrapperPass>().getDomTree(),150 ConstHoistWithBlockFrequency151 ? &getAnalysis<BlockFrequencyInfoWrapperPass>().getBFI()152 : nullptr,153 Fn.getEntryBlock(),154 &getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI());155 156 LLVM_DEBUG(dbgs() << "********** End Constant Hoisting **********\n");157 158 return MadeChange;159}160 161void ConstantHoistingPass::collectMatInsertPts(162 const RebasedConstantListType &RebasedConstants,163 SmallVectorImpl<BasicBlock::iterator> &MatInsertPts) const {164 for (const RebasedConstantInfo &RCI : RebasedConstants)165 for (const ConstantUser &U : RCI.Uses)166 MatInsertPts.emplace_back(findMatInsertPt(U.Inst, U.OpndIdx));167}168 169/// Find the constant materialization insertion point.170BasicBlock::iterator ConstantHoistingPass::findMatInsertPt(Instruction *Inst,171 unsigned Idx) const {172 // If the operand is a cast instruction, then we have to materialize the173 // constant before the cast instruction.174 if (Idx != ~0U) {175 Value *Opnd = Inst->getOperand(Idx);176 if (auto CastInst = dyn_cast<Instruction>(Opnd))177 if (CastInst->isCast())178 return CastInst->getIterator();179 }180 181 // The simple and common case. This also includes constant expressions.182 if (!isa<PHINode>(Inst) && !Inst->isEHPad())183 return Inst->getIterator();184 185 // We can't insert directly before a phi node or an eh pad. Insert before186 // the terminator of the incoming or dominating block.187 assert(Entry != Inst->getParent() && "PHI or landing pad in entry block!");188 BasicBlock *InsertionBlock = nullptr;189 if (Idx != ~0U && isa<PHINode>(Inst)) {190 InsertionBlock = cast<PHINode>(Inst)->getIncomingBlock(Idx);191 if (!InsertionBlock->isEHPad()) {192 return InsertionBlock->getTerminator()->getIterator();193 }194 } else {195 InsertionBlock = Inst->getParent();196 }197 198 // This must be an EH pad. Iterate over immediate dominators until we find a199 // non-EH pad. We need to skip over catchswitch blocks, which are both EH pads200 // and terminators.201 auto *IDom = DT->getNode(InsertionBlock)->getIDom();202 while (IDom->getBlock()->isEHPad()) {203 assert(Entry != IDom->getBlock() && "eh pad in entry block");204 IDom = IDom->getIDom();205 }206 207 return IDom->getBlock()->getTerminator()->getIterator();208}209 210/// Given \p BBs as input, find another set of BBs which collectively211/// dominates \p BBs and have the minimal sum of frequencies. Return the BB212/// set found in \p BBs.213static void findBestInsertionSet(DominatorTree &DT, BlockFrequencyInfo &BFI,214 BasicBlock *Entry,215 SetVector<BasicBlock *> &BBs) {216 assert(!BBs.count(Entry) && "Assume Entry is not in BBs");217 // Nodes on the current path to the root.218 SmallPtrSet<BasicBlock *, 8> Path;219 // Candidates includes any block 'BB' in set 'BBs' that is not strictly220 // dominated by any other blocks in set 'BBs', and all nodes in the path221 // in the dominator tree from Entry to 'BB'.222 SmallPtrSet<BasicBlock *, 16> Candidates;223 for (auto *BB : BBs) {224 // Ignore unreachable basic blocks.225 if (!DT.isReachableFromEntry(BB))226 continue;227 Path.clear();228 // Walk up the dominator tree until Entry or another BB in BBs229 // is reached. Insert the nodes on the way to the Path.230 BasicBlock *Node = BB;231 // The "Path" is a candidate path to be added into Candidates set.232 bool isCandidate = false;233 do {234 Path.insert(Node);235 if (Node == Entry || Candidates.count(Node)) {236 isCandidate = true;237 break;238 }239 assert(DT.getNode(Node)->getIDom() &&240 "Entry doens't dominate current Node");241 Node = DT.getNode(Node)->getIDom()->getBlock();242 } while (!BBs.count(Node));243 244 // If isCandidate is false, Node is another Block in BBs dominating245 // current 'BB'. Drop the nodes on the Path.246 if (!isCandidate)247 continue;248 249 // Add nodes on the Path into Candidates.250 Candidates.insert_range(Path);251 }252 253 // Sort the nodes in Candidates in top-down order and save the nodes254 // in Orders.255 unsigned Idx = 0;256 SmallVector<BasicBlock *, 16> Orders;257 Orders.push_back(Entry);258 while (Idx != Orders.size()) {259 BasicBlock *Node = Orders[Idx++];260 for (auto *ChildDomNode : DT.getNode(Node)->children()) {261 if (Candidates.count(ChildDomNode->getBlock()))262 Orders.push_back(ChildDomNode->getBlock());263 }264 }265 266 // Visit Orders in bottom-up order.267 using InsertPtsCostPair =268 std::pair<SetVector<BasicBlock *>, BlockFrequency>;269 270 // InsertPtsMap is a map from a BB to the best insertion points for the271 // subtree of BB (subtree not including the BB itself).272 DenseMap<BasicBlock *, InsertPtsCostPair> InsertPtsMap;273 InsertPtsMap.reserve(Orders.size() + 1);274 for (BasicBlock *Node : llvm::reverse(Orders)) {275 bool NodeInBBs = BBs.count(Node);276 auto &[InsertPts, InsertPtsFreq] = InsertPtsMap[Node];277 278 // Return the optimal insert points in BBs.279 if (Node == Entry) {280 BBs.clear();281 if (InsertPtsFreq > BFI.getBlockFreq(Node) ||282 (InsertPtsFreq == BFI.getBlockFreq(Node) && InsertPts.size() > 1))283 BBs.insert(Entry);284 else285 BBs.insert_range(InsertPts);286 break;287 }288 289 BasicBlock *Parent = DT.getNode(Node)->getIDom()->getBlock();290 // Initially, ParentInsertPts is empty and ParentPtsFreq is 0. Every child291 // will update its parent's ParentInsertPts and ParentPtsFreq.292 auto &[ParentInsertPts, ParentPtsFreq] = InsertPtsMap[Parent];293 // Choose to insert in Node or in subtree of Node.294 // Don't hoist to EHPad because we may not find a proper place to insert295 // in EHPad.296 // If the total frequency of InsertPts is the same as the frequency of the297 // target Node, and InsertPts contains more than one nodes, choose hoisting298 // to reduce code size.299 if (NodeInBBs ||300 (!Node->isEHPad() &&301 (InsertPtsFreq > BFI.getBlockFreq(Node) ||302 (InsertPtsFreq == BFI.getBlockFreq(Node) && InsertPts.size() > 1)))) {303 ParentInsertPts.insert(Node);304 ParentPtsFreq += BFI.getBlockFreq(Node);305 } else {306 ParentInsertPts.insert_range(InsertPts);307 ParentPtsFreq += InsertPtsFreq;308 }309 }310}311 312/// Find an insertion point that dominates all uses.313SetVector<BasicBlock::iterator>314ConstantHoistingPass::findConstantInsertionPoint(315 const ConstantInfo &ConstInfo,316 const ArrayRef<BasicBlock::iterator> MatInsertPts) const {317 assert(!ConstInfo.RebasedConstants.empty() && "Invalid constant info entry.");318 // Collect all basic blocks.319 SetVector<BasicBlock *> BBs;320 SetVector<BasicBlock::iterator> InsertPts;321 322 for (BasicBlock::iterator MatInsertPt : MatInsertPts)323 BBs.insert(MatInsertPt->getParent());324 325 if (BBs.count(Entry)) {326 InsertPts.insert(Entry->begin());327 return InsertPts;328 }329 330 if (BFI) {331 findBestInsertionSet(*DT, *BFI, Entry, BBs);332 for (BasicBlock *BB : BBs)333 InsertPts.insert(BB->getFirstInsertionPt());334 return InsertPts;335 }336 337 while (BBs.size() >= 2) {338 BasicBlock *BB, *BB1, *BB2;339 BB1 = BBs.pop_back_val();340 BB2 = BBs.pop_back_val();341 BB = DT->findNearestCommonDominator(BB1, BB2);342 if (BB == Entry) {343 InsertPts.insert(Entry->begin());344 return InsertPts;345 }346 BBs.insert(BB);347 }348 assert((BBs.size() == 1) && "Expected only one element.");349 Instruction &FirstInst = (*BBs.begin())->front();350 InsertPts.insert(findMatInsertPt(&FirstInst));351 return InsertPts;352}353 354/// Record constant integer ConstInt for instruction Inst at operand355/// index Idx.356///357/// The operand at index Idx is not necessarily the constant integer itself. It358/// could also be a cast instruction or a constant expression that uses the359/// constant integer.360void ConstantHoistingPass::collectConstantCandidates(361 ConstCandMapType &ConstCandMap, Instruction *Inst, unsigned Idx,362 ConstantInt *ConstInt) {363 if (ConstInt->getType()->isVectorTy())364 return;365 366 InstructionCost Cost;367 // Ask the target about the cost of materializing the constant for the given368 // instruction and operand index.369 if (auto IntrInst = dyn_cast<IntrinsicInst>(Inst))370 Cost = TTI->getIntImmCostIntrin(IntrInst->getIntrinsicID(), Idx,371 ConstInt->getValue(), ConstInt->getType(),372 TargetTransformInfo::TCK_SizeAndLatency);373 else374 Cost = TTI->getIntImmCostInst(375 Inst->getOpcode(), Idx, ConstInt->getValue(), ConstInt->getType(),376 TargetTransformInfo::TCK_SizeAndLatency, Inst);377 378 // Ignore cheap integer constants.379 if (Cost > TargetTransformInfo::TCC_Basic) {380 ConstCandMapType::iterator Itr;381 bool Inserted;382 ConstPtrUnionType Cand = ConstInt;383 std::tie(Itr, Inserted) = ConstCandMap.try_emplace(Cand);384 if (Inserted) {385 ConstIntCandVec.push_back(ConstantCandidate(ConstInt));386 Itr->second = ConstIntCandVec.size() - 1;387 }388 ConstIntCandVec[Itr->second].addUser(Inst, Idx, Cost.getValue());389 LLVM_DEBUG(if (isa<ConstantInt>(Inst->getOperand(Idx))) dbgs()390 << "Collect constant " << *ConstInt << " from " << *Inst391 << " with cost " << Cost << '\n';392 else dbgs() << "Collect constant " << *ConstInt393 << " indirectly from " << *Inst << " via "394 << *Inst->getOperand(Idx) << " with cost " << Cost395 << '\n';);396 }397}398 399/// Record constant GEP expression for instruction Inst at operand index Idx.400void ConstantHoistingPass::collectConstantCandidates(401 ConstCandMapType &ConstCandMap, Instruction *Inst, unsigned Idx,402 ConstantExpr *ConstExpr) {403 // TODO: Handle vector GEPs404 if (ConstExpr->getType()->isVectorTy())405 return;406 407 GlobalVariable *BaseGV = dyn_cast<GlobalVariable>(ConstExpr->getOperand(0));408 if (!BaseGV)409 return;410 411 // Get offset from the base GV.412 PointerType *GVPtrTy = cast<PointerType>(BaseGV->getType());413 IntegerType *OffsetTy = DL->getIndexType(*Ctx, GVPtrTy->getAddressSpace());414 APInt Offset(DL->getTypeSizeInBits(OffsetTy), /*val*/ 0, /*isSigned*/ true);415 auto *GEPO = cast<GEPOperator>(ConstExpr);416 417 // TODO: If we have a mix of inbounds and non-inbounds GEPs, then basing a418 // non-inbounds GEP on an inbounds GEP is potentially incorrect. Restrict to419 // inbounds GEP for now -- alternatively, we could drop inbounds from the420 // constant expression,421 if (!GEPO->isInBounds())422 return;423 424 if (!GEPO->accumulateConstantOffset(*DL, Offset))425 return;426 427 if (!Offset.isIntN(32))428 return;429 430 // A constant GEP expression that has a GlobalVariable as base pointer is431 // usually lowered to a load from constant pool. Such operation is unlikely432 // to be cheaper than compute it by <Base + Offset>, which can be lowered to433 // an ADD instruction or folded into Load/Store instruction.434 InstructionCost Cost =435 TTI->getIntImmCostInst(Instruction::Add, 1, Offset, OffsetTy,436 TargetTransformInfo::TCK_SizeAndLatency, Inst);437 ConstCandVecType &ExprCandVec = ConstGEPCandMap[BaseGV];438 ConstCandMapType::iterator Itr;439 bool Inserted;440 ConstPtrUnionType Cand = ConstExpr;441 std::tie(Itr, Inserted) = ConstCandMap.try_emplace(Cand);442 if (Inserted) {443 ExprCandVec.push_back(ConstantCandidate(444 ConstantInt::get(Type::getInt32Ty(*Ctx), Offset.getLimitedValue()),445 ConstExpr));446 Itr->second = ExprCandVec.size() - 1;447 }448 ExprCandVec[Itr->second].addUser(Inst, Idx, Cost.getValue());449}450 451/// Check the operand for instruction Inst at index Idx.452void ConstantHoistingPass::collectConstantCandidates(453 ConstCandMapType &ConstCandMap, Instruction *Inst, unsigned Idx) {454 Value *Opnd = Inst->getOperand(Idx);455 456 // Visit constant integers.457 if (auto ConstInt = dyn_cast<ConstantInt>(Opnd)) {458 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);459 return;460 }461 462 // Visit cast instructions that have constant integers.463 if (auto CastInst = dyn_cast<Instruction>(Opnd)) {464 // Only visit cast instructions, which have been skipped. All other465 // instructions should have already been visited.466 if (!CastInst->isCast())467 return;468 469 if (auto *ConstInt = dyn_cast<ConstantInt>(CastInst->getOperand(0))) {470 // Pretend the constant is directly used by the instruction and ignore471 // the cast instruction.472 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);473 return;474 }475 }476 477 // Visit constant expressions that have constant integers.478 if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) {479 // Handle constant gep expressions.480 if (ConstHoistGEP && isa<GEPOperator>(ConstExpr))481 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstExpr);482 483 // Only visit constant cast expressions.484 if (!ConstExpr->isCast())485 return;486 487 if (auto ConstInt = dyn_cast<ConstantInt>(ConstExpr->getOperand(0))) {488 // Pretend the constant is directly used by the instruction and ignore489 // the constant expression.490 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);491 return;492 }493 }494}495 496/// Scan the instruction for expensive integer constants and record them497/// in the constant candidate vector.498void ConstantHoistingPass::collectConstantCandidates(499 ConstCandMapType &ConstCandMap, Instruction *Inst) {500 // Skip all cast instructions. They are visited indirectly later on.501 if (Inst->isCast())502 return;503 504 // Scan all operands.505 for (unsigned Idx = 0, E = Inst->getNumOperands(); Idx != E; ++Idx) {506 // The cost of materializing the constants (defined in507 // `TargetTransformInfo::getIntImmCostInst`) for instructions which only508 // take constant variables is lower than `TargetTransformInfo::TCC_Basic`.509 // So it's safe for us to collect constant candidates from all510 // IntrinsicInsts.511 if (canReplaceOperandWithVariable(Inst, Idx)) {512 collectConstantCandidates(ConstCandMap, Inst, Idx);513 }514 } // end of for all operands515}516 517/// Collect all integer constants in the function that cannot be folded518/// into an instruction itself.519void ConstantHoistingPass::collectConstantCandidates(Function &Fn) {520 ConstCandMapType ConstCandMap;521 for (BasicBlock &BB : Fn) {522 // Ignore unreachable basic blocks.523 if (!DT->isReachableFromEntry(&BB))524 continue;525 for (Instruction &Inst : BB)526 if (!TTI->preferToKeepConstantsAttached(Inst, Fn))527 collectConstantCandidates(ConstCandMap, &Inst);528 }529}530 531// From a list of constants, one needs to picked as the base and the other532// constants will be transformed into an offset from that base constant. The533// question is which we can pick best? For example, consider these constants534// and their number of uses:535//536// Constants| 2 | 4 | 12 | 42 |537// NumUses | 3 | 2 | 8 | 7 |538//539// Selecting constant 12 because it has the most uses will generate negative540// offsets for constants 2 and 4 (i.e. -10 and -8 respectively). If negative541// offsets lead to less optimal code generation, then there might be better542// solutions. Suppose immediates in the range of 0..35 are most optimally543// supported by the architecture, then selecting constant 2 is most optimal544// because this will generate offsets: 0, 2, 10, 40. Offsets 0, 2 and 10 are in545// range 0..35, and thus 3 + 2 + 8 = 13 uses are in range. Selecting 12 would546// have only 8 uses in range, so choosing 2 as a base is more optimal. Thus, in547// selecting the base constant the range of the offsets is a very important548// factor too that we take into account here. This algorithm calculates a total549// costs for selecting a constant as the base and substract the costs if550// immediates are out of range. It has quadratic complexity, so we call this551// function only when we're optimising for size and there are less than 100552// constants, we fall back to the straightforward algorithm otherwise553// which does not do all the offset calculations.554unsigned555ConstantHoistingPass::maximizeConstantsInRange(ConstCandVecType::iterator S,556 ConstCandVecType::iterator E,557 ConstCandVecType::iterator &MaxCostItr) {558 unsigned NumUses = 0;559 560 if (!OptForSize || std::distance(S,E) > 100) {561 for (auto ConstCand = S; ConstCand != E; ++ConstCand) {562 NumUses += ConstCand->Uses.size();563 if (ConstCand->CumulativeCost > MaxCostItr->CumulativeCost)564 MaxCostItr = ConstCand;565 }566 return NumUses;567 }568 569 LLVM_DEBUG(dbgs() << "== Maximize constants in range ==\n");570 InstructionCost MaxCost = -1;571 for (auto ConstCand = S; ConstCand != E; ++ConstCand) {572 auto Value = ConstCand->ConstInt->getValue();573 Type *Ty = ConstCand->ConstInt->getType();574 InstructionCost Cost = 0;575 NumUses += ConstCand->Uses.size();576 LLVM_DEBUG(dbgs() << "= Constant: " << ConstCand->ConstInt->getValue()577 << "\n");578 579 for (auto User : ConstCand->Uses) {580 unsigned Opcode = User.Inst->getOpcode();581 unsigned OpndIdx = User.OpndIdx;582 Cost += TTI->getIntImmCostInst(Opcode, OpndIdx, Value, Ty,583 TargetTransformInfo::TCK_SizeAndLatency);584 LLVM_DEBUG(dbgs() << "Cost: " << Cost << "\n");585 586 for (auto C2 = S; C2 != E; ++C2) {587 APInt Diff = C2->ConstInt->getValue() - ConstCand->ConstInt->getValue();588 const InstructionCost ImmCosts =589 TTI->getIntImmCodeSizeCost(Opcode, OpndIdx, Diff, Ty);590 Cost -= ImmCosts;591 LLVM_DEBUG(dbgs() << "Offset " << Diff << " "592 << "has penalty: " << ImmCosts << "\n"593 << "Adjusted cost: " << Cost << "\n");594 }595 }596 LLVM_DEBUG(dbgs() << "Cumulative cost: " << Cost << "\n");597 if (Cost > MaxCost) {598 MaxCost = Cost;599 MaxCostItr = ConstCand;600 LLVM_DEBUG(dbgs() << "New candidate: " << MaxCostItr->ConstInt->getValue()601 << "\n");602 }603 }604 return NumUses;605}606 607/// Find the base constant within the given range and rebase all other608/// constants with respect to the base constant.609void ConstantHoistingPass::findAndMakeBaseConstant(610 ConstCandVecType::iterator S, ConstCandVecType::iterator E,611 SmallVectorImpl<consthoist::ConstantInfo> &ConstInfoVec) {612 auto MaxCostItr = S;613 unsigned NumUses = maximizeConstantsInRange(S, E, MaxCostItr);614 615 // Don't hoist constants that have only one use.616 if (NumUses <= 1)617 return;618 619 ConstantInt *ConstInt = MaxCostItr->ConstInt;620 ConstantExpr *ConstExpr = MaxCostItr->ConstExpr;621 ConstantInfo ConstInfo;622 ConstInfo.BaseInt = ConstInt;623 ConstInfo.BaseExpr = ConstExpr;624 Type *Ty = ConstInt->getType();625 626 // Rebase the constants with respect to the base constant.627 for (auto ConstCand = S; ConstCand != E; ++ConstCand) {628 APInt Diff = ConstCand->ConstInt->getValue() - ConstInt->getValue();629 Constant *Offset = Diff == 0 ? nullptr : ConstantInt::get(Ty, Diff);630 Type *ConstTy =631 ConstCand->ConstExpr ? ConstCand->ConstExpr->getType() : nullptr;632 ConstInfo.RebasedConstants.push_back(633 RebasedConstantInfo(std::move(ConstCand->Uses), Offset, ConstTy));634 }635 ConstInfoVec.push_back(std::move(ConstInfo));636}637 638/// Finds and combines constant candidates that can be easily639/// rematerialized with an add from a common base constant.640void ConstantHoistingPass::findBaseConstants(GlobalVariable *BaseGV) {641 // If BaseGV is nullptr, find base among candidate constant integers;642 // Otherwise find base among constant GEPs that share the same BaseGV.643 ConstCandVecType &ConstCandVec = BaseGV ?644 ConstGEPCandMap[BaseGV] : ConstIntCandVec;645 ConstInfoVecType &ConstInfoVec = BaseGV ?646 ConstGEPInfoMap[BaseGV] : ConstIntInfoVec;647 648 // Sort the constants by value and type. This invalidates the mapping!649 llvm::stable_sort(ConstCandVec, [](const ConstantCandidate &LHS,650 const ConstantCandidate &RHS) {651 if (LHS.ConstInt->getType() != RHS.ConstInt->getType())652 return LHS.ConstInt->getBitWidth() < RHS.ConstInt->getBitWidth();653 return LHS.ConstInt->getValue().ult(RHS.ConstInt->getValue());654 });655 656 // Simple linear scan through the sorted constant candidate vector for viable657 // merge candidates.658 auto MinValItr = ConstCandVec.begin();659 for (auto CC = std::next(ConstCandVec.begin()), E = ConstCandVec.end();660 CC != E; ++CC) {661 if (MinValItr->ConstInt->getType() == CC->ConstInt->getType()) {662 Type *MemUseValTy = nullptr;663 for (auto &U : CC->Uses) {664 auto *UI = U.Inst;665 if (LoadInst *LI = dyn_cast<LoadInst>(UI)) {666 MemUseValTy = LI->getType();667 break;668 } else if (StoreInst *SI = dyn_cast<StoreInst>(UI)) {669 // Make sure the constant is used as pointer operand of the StoreInst.670 if (SI->getPointerOperand() == SI->getOperand(U.OpndIdx)) {671 MemUseValTy = SI->getValueOperand()->getType();672 break;673 }674 }675 }676 677 // Check if the constant is in range of an add with immediate.678 APInt Diff = CC->ConstInt->getValue() - MinValItr->ConstInt->getValue();679 if ((Diff.getBitWidth() <= 64) &&680 TTI->isLegalAddImmediate(Diff.getSExtValue()) &&681 // Check if Diff can be used as offset in addressing mode of the user682 // memory instruction.683 (!MemUseValTy || TTI->isLegalAddressingMode(MemUseValTy,684 /*BaseGV*/nullptr, /*BaseOffset*/Diff.getSExtValue(),685 /*HasBaseReg*/true, /*Scale*/0)))686 continue;687 }688 // We either have now a different constant type or the constant is not in689 // range of an add with immediate anymore.690 findAndMakeBaseConstant(MinValItr, CC, ConstInfoVec);691 // Start a new base constant search.692 MinValItr = CC;693 }694 // Finalize the last base constant search.695 findAndMakeBaseConstant(MinValItr, ConstCandVec.end(), ConstInfoVec);696}697 698/// Updates the operand at Idx in instruction Inst with the result of699/// instruction Mat. If the instruction is a PHI node then special700/// handling for duplicate values from the same incoming basic block is701/// required.702/// \return The update will always succeed, but the return value indicated if703/// Mat was used for the update or not.704static bool updateOperand(Instruction *Inst, unsigned Idx, Instruction *Mat) {705 if (auto PHI = dyn_cast<PHINode>(Inst)) {706 // Check if any previous operand of the PHI node has the same incoming basic707 // block. This is a very odd case that happens when the incoming basic block708 // has a switch statement. In this case use the same value as the previous709 // operand(s), otherwise we will fail verification due to different values.710 // The values are actually the same, but the variable names are different711 // and the verifier doesn't like that.712 BasicBlock *IncomingBB = PHI->getIncomingBlock(Idx);713 for (unsigned i = 0; i < Idx; ++i) {714 if (PHI->getIncomingBlock(i) == IncomingBB) {715 Value *IncomingVal = PHI->getIncomingValue(i);716 Inst->setOperand(Idx, IncomingVal);717 return false;718 }719 }720 }721 722 Inst->setOperand(Idx, Mat);723 return true;724}725 726/// Emit materialization code for all rebased constants and update their727/// users.728void ConstantHoistingPass::emitBaseConstants(Instruction *Base,729 UserAdjustment *Adj) {730 Instruction *Mat = Base;731 732 // The same offset can be dereferenced to different types in nested struct.733 if (!Adj->Offset && Adj->Ty && Adj->Ty != Base->getType())734 Adj->Offset = ConstantInt::get(Type::getInt32Ty(*Ctx), 0);735 736 if (Adj->Offset) {737 if (Adj->Ty) {738 // Constant being rebased is a ConstantExpr.739 Mat = GetElementPtrInst::Create(Type::getInt8Ty(*Ctx), Base, Adj->Offset,740 "mat_gep", Adj->MatInsertPt);741 // Hide it behind a bitcast.742 Mat = new BitCastInst(Mat, Adj->Ty, "mat_bitcast",743 Adj->MatInsertPt->getIterator());744 } else745 // Constant being rebased is a ConstantInt.746 Mat =747 BinaryOperator::Create(Instruction::Add, Base, Adj->Offset,748 "const_mat", Adj->MatInsertPt->getIterator());749 750 LLVM_DEBUG(dbgs() << "Materialize constant (" << *Base->getOperand(0)751 << " + " << *Adj->Offset << ") in BB "752 << Mat->getParent()->getName() << '\n'753 << *Mat << '\n');754 Mat->setDebugLoc(Adj->User.Inst->getDebugLoc());755 }756 Value *Opnd = Adj->User.Inst->getOperand(Adj->User.OpndIdx);757 758 // Visit constant integer.759 if (isa<ConstantInt>(Opnd)) {760 LLVM_DEBUG(dbgs() << "Update: " << *Adj->User.Inst << '\n');761 if (!updateOperand(Adj->User.Inst, Adj->User.OpndIdx, Mat) && Adj->Offset)762 Mat->eraseFromParent();763 LLVM_DEBUG(dbgs() << "To : " << *Adj->User.Inst << '\n');764 return;765 }766 767 // Visit cast instruction.768 if (auto CastInst = dyn_cast<Instruction>(Opnd)) {769 assert(CastInst->isCast() && "Expected an cast instruction!");770 // Check if we already have visited this cast instruction before to avoid771 // unnecessary cloning.772 Instruction *&ClonedCastInst = ClonedCastMap[CastInst];773 if (!ClonedCastInst) {774 ClonedCastInst = CastInst->clone();775 ClonedCastInst->setOperand(0, Mat);776 ClonedCastInst->insertAfter(CastInst->getIterator());777 // Use the same debug location as the original cast instruction.778 ClonedCastInst->setDebugLoc(CastInst->getDebugLoc());779 LLVM_DEBUG(dbgs() << "Clone instruction: " << *CastInst << '\n'780 << "To : " << *ClonedCastInst << '\n');781 }782 783 LLVM_DEBUG(dbgs() << "Update: " << *Adj->User.Inst << '\n');784 updateOperand(Adj->User.Inst, Adj->User.OpndIdx, ClonedCastInst);785 LLVM_DEBUG(dbgs() << "To : " << *Adj->User.Inst << '\n');786 return;787 }788 789 // Visit constant expression.790 if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) {791 if (isa<GEPOperator>(ConstExpr)) {792 // Operand is a ConstantGEP, replace it.793 updateOperand(Adj->User.Inst, Adj->User.OpndIdx, Mat);794 return;795 }796 797 // Aside from constant GEPs, only constant cast expressions are collected.798 assert(ConstExpr->isCast() && "ConstExpr should be a cast");799 Instruction *ConstExprInst = ConstExpr->getAsInstruction();800 ConstExprInst->insertBefore(Adj->MatInsertPt);801 ConstExprInst->setOperand(0, Mat);802 803 // Use the same debug location as the instruction we are about to update.804 ConstExprInst->setDebugLoc(Adj->User.Inst->getDebugLoc());805 806 LLVM_DEBUG(dbgs() << "Create instruction: " << *ConstExprInst << '\n'807 << "From : " << *ConstExpr << '\n');808 LLVM_DEBUG(dbgs() << "Update: " << *Adj->User.Inst << '\n');809 if (!updateOperand(Adj->User.Inst, Adj->User.OpndIdx, ConstExprInst)) {810 ConstExprInst->eraseFromParent();811 if (Adj->Offset)812 Mat->eraseFromParent();813 }814 LLVM_DEBUG(dbgs() << "To : " << *Adj->User.Inst << '\n');815 return;816 }817}818 819/// Hoist and hide the base constant behind a bitcast and emit820/// materialization code for derived constants.821bool ConstantHoistingPass::emitBaseConstants(GlobalVariable *BaseGV) {822 bool MadeChange = false;823 SmallVectorImpl<consthoist::ConstantInfo> &ConstInfoVec =824 BaseGV ? ConstGEPInfoMap[BaseGV] : ConstIntInfoVec;825 for (const consthoist::ConstantInfo &ConstInfo : ConstInfoVec) {826 SmallVector<BasicBlock::iterator, 4> MatInsertPts;827 collectMatInsertPts(ConstInfo.RebasedConstants, MatInsertPts);828 SetVector<BasicBlock::iterator> IPSet =829 findConstantInsertionPoint(ConstInfo, MatInsertPts);830 // We can have an empty set if the function contains unreachable blocks.831 if (IPSet.empty())832 continue;833 834 unsigned UsesNum = 0;835 unsigned ReBasesNum = 0;836 unsigned NotRebasedNum = 0;837 for (const BasicBlock::iterator &IP : IPSet) {838 // First, collect constants depending on this IP of the base.839 UsesNum = 0;840 SmallVector<UserAdjustment, 4> ToBeRebased;841 unsigned MatCtr = 0;842 for (auto const &RCI : ConstInfo.RebasedConstants) {843 UsesNum += RCI.Uses.size();844 for (auto const &U : RCI.Uses) {845 const BasicBlock::iterator &MatInsertPt = MatInsertPts[MatCtr++];846 BasicBlock *OrigMatInsertBB = MatInsertPt->getParent();847 // If Base constant is to be inserted in multiple places,848 // generate rebase for U using the Base dominating U.849 if (IPSet.size() == 1 ||850 DT->dominates(IP->getParent(), OrigMatInsertBB))851 ToBeRebased.emplace_back(RCI.Offset, RCI.Ty, MatInsertPt, U);852 }853 }854 855 // If only few constants depend on this IP of base, skip rebasing,856 // assuming the base and the rebased have the same materialization cost.857 if (ToBeRebased.size() < MinNumOfDependentToRebase) {858 NotRebasedNum += ToBeRebased.size();859 continue;860 }861 862 // Emit an instance of the base at this IP.863 Instruction *Base = nullptr;864 // Hoist and hide the base constant behind a bitcast.865 if (ConstInfo.BaseExpr) {866 assert(BaseGV && "A base constant expression must have an base GV");867 Type *Ty = ConstInfo.BaseExpr->getType();868 Base = new BitCastInst(ConstInfo.BaseExpr, Ty, "const", IP);869 } else {870 IntegerType *Ty = ConstInfo.BaseInt->getIntegerType();871 Base = new BitCastInst(ConstInfo.BaseInt, Ty, "const", IP);872 }873 874 Base->setDebugLoc(IP->getDebugLoc());875 876 LLVM_DEBUG(dbgs() << "Hoist constant (" << *ConstInfo.BaseInt877 << ") to BB " << IP->getParent()->getName() << '\n'878 << *Base << '\n');879 880 // Emit materialization code for rebased constants depending on this IP.881 for (UserAdjustment &R : ToBeRebased) {882 emitBaseConstants(Base, &R);883 ReBasesNum++;884 // Use the same debug location as the last user of the constant.885 Base->setDebugLoc(DebugLoc::getMergedLocation(886 Base->getDebugLoc(), R.User.Inst->getDebugLoc()));887 }888 assert(!Base->use_empty() && "The use list is empty!?");889 assert(isa<Instruction>(Base->user_back()) &&890 "All uses should be instructions.");891 }892 (void)UsesNum;893 (void)ReBasesNum;894 (void)NotRebasedNum;895 // Expect all uses are rebased after rebase is done.896 assert(UsesNum == (ReBasesNum + NotRebasedNum) &&897 "Not all uses are rebased");898 899 NumConstantsHoisted++;900 901 // Base constant is also included in ConstInfo.RebasedConstants, so902 // deduct 1 from ConstInfo.RebasedConstants.size().903 NumConstantsRebased += ConstInfo.RebasedConstants.size() - 1;904 905 MadeChange = true;906 }907 return MadeChange;908}909 910/// Check all cast instructions we made a copy of and remove them if they911/// have no more users.912void ConstantHoistingPass::deleteDeadCastInst() const {913 for (auto const &I : ClonedCastMap)914 if (I.first->use_empty())915 I.first->eraseFromParent();916}917 918/// Optimize expensive integer constants in the given function.919bool ConstantHoistingPass::runImpl(Function &Fn, TargetTransformInfo &TTI,920 DominatorTree &DT, BlockFrequencyInfo *BFI,921 BasicBlock &Entry, ProfileSummaryInfo *PSI) {922 this->TTI = &TTI;923 this->DT = &DT;924 this->BFI = BFI;925 this->DL = &Fn.getDataLayout();926 this->Ctx = &Fn.getContext();927 this->Entry = &Entry;928 this->PSI = PSI;929 this->OptForSize = llvm::shouldOptimizeForSize(Entry.getParent(), PSI, BFI,930 PGSOQueryType::IRPass);931 932 // Collect all constant candidates.933 collectConstantCandidates(Fn);934 935 // Combine constants that can be easily materialized with an add from a common936 // base constant.937 if (!ConstIntCandVec.empty())938 findBaseConstants(nullptr);939 for (const auto &MapEntry : ConstGEPCandMap)940 if (!MapEntry.second.empty())941 findBaseConstants(MapEntry.first);942 943 // Finally hoist the base constant and emit materialization code for dependent944 // constants.945 bool MadeChange = false;946 if (!ConstIntInfoVec.empty())947 MadeChange = emitBaseConstants(nullptr);948 for (const auto &MapEntry : ConstGEPInfoMap)949 if (!MapEntry.second.empty())950 MadeChange |= emitBaseConstants(MapEntry.first);951 952 953 // Cleanup dead instructions.954 deleteDeadCastInst();955 956 cleanup();957 958 return MadeChange;959}960 961PreservedAnalyses ConstantHoistingPass::run(Function &F,962 FunctionAnalysisManager &AM) {963 auto &DT = AM.getResult<DominatorTreeAnalysis>(F);964 auto &TTI = AM.getResult<TargetIRAnalysis>(F);965 auto BFI = ConstHoistWithBlockFrequency966 ? &AM.getResult<BlockFrequencyAnalysis>(F)967 : nullptr;968 auto &MAMProxy = AM.getResult<ModuleAnalysisManagerFunctionProxy>(F);969 auto *PSI = MAMProxy.getCachedResult<ProfileSummaryAnalysis>(*F.getParent());970 if (!runImpl(F, TTI, DT, BFI, F.getEntryBlock(), PSI))971 return PreservedAnalyses::all();972 973 PreservedAnalyses PA;974 PA.preserveSet<CFGAnalyses>();975 return PA;976}977